The present invention provides rare earth compounds exhibiting photo-luminescent properties when irradiated, apparatus, and methods for their use. Specifically, the invention provides rare earth compounds that produce continuous broadband, super-radiant, and discrete line emissions by up-conversion and/or down-conversion of the incident light. The broadband emission may be visible.
Wavelength up-conversion and down-conversion are known phenomena. Any material with radiative transitions between more than two energy levels could be used, in principle, as an up- and as a down-converter. Such a material would emit light at discrete wavelengths corresponding to the radiative energy level transitions.
Up-conversion comes from inducing successive absorption in a single ion. This can lead to up-conversion emission through a scheme termed Excited State Absorption (ESA) while other schemes involve two and three phonon assisted energy transfers such as in Ground State Absorption (GSA), Energy Transfer Up-conversion (ETU), and Addition of photon by Transfer of Energy (APTE). Up-conversion by sequential ground-state excitation/excited state excitation (GSA/ESA) involves a two step excitation of electrons from the valence band via an intermediate level into the conduction band by absorption of two low energy photons followed by a radiative transition of the electron from the conduction band into the valence band.
Down-conversion involves the reverse processes, i.e. absorption of one high-energy photon by a band-band transition and the subsequent transition of the electron into the valence band in a two-step process via the intermediate level, with one low-energy photon emitted in each intermediate step.
Many infrared-to-visible up-conversion materials are known to emit light at a discrete visible wavelength. Some of these materials are commonly used in laser applications, others as phosphors, to convert infrared emissions to visible laser and light emissions. Most of these materials consist of crystals or glasses that contain one or more dopants of rare earth element compounds.
Trivalent ytterbium ion (Yb3+) has been used as a sensitizer of energy transfer for infrared to visible up-conversion laser. There are only two known energy levels for trivalent ytterbium, the 2F7/2 ground state and 2F5/2 excitation state. Since there are only two known energy levels for Yb3+, it would not be expected to produce up-converted or down-converted luminescence alone. Instead, Yb3+ has commonly been used as a sensitizer for at least one other rare earth ion. The electron transition between 2F7/2 and 2F5/2 occurs at about 104 cm−1 energy level or at about 1,000 nm wavelength of light. The Yb3+ absorbs energy at about 976 nm. Under pumping of an infrared laser diode, Yb3+ ion at ground state is excited to 2F5/2 excitation state. The excited Yb3+ transfers its energy to a nearby rare earth ion, such as a Tm3+, Ho3+, or Er3+ ion, which then emits visible light through other known electron transitions.
Down-conversion is a more common phenomenon. As an example, fluorescent light devices commonly contain mercury vapor which when electrically excited emits ultraviolet light at about 253 nm wavelength. High energy ultraviolet photons are down-converted into the visible region of the light spectrum by interaction of the UV wavelength light with down-converting phosphors coated on the inside of the fluorescent glass tubes. In addition, some visible LED devices are based upon a gallium nitride ultraviolet emitter using down conversion phosphors to generate white light.
Solid state devices that emit infrared light are generally of lower cost than similar devices that emit visible or ultraviolet light. It would be an advancement in the art to provide materials that efficiently up-convert infrared light such that low-cost light sources may be used to produce more valuable visible and UV light. It would also be an advancement in the art to provide materials that efficiently down-convert UV light to broadband or desired discrete wavelength luminescent light.
As mentioned above, currently known up-converting luminescent materials emit light at discrete wavelengths. It will be appreciated that broadband emissions, especially in the visible region, would have valuable applications for general lighting applications and for tunable laser applications.